The present invention relates to automatic welding equipment having a welding torch which is advanced along a weld seam during a welding operation by a traveling carriage riding on a track member; and more particularly, the present invention relates to apparatus for providing accurate tracking of the weld seam by the welding torch as it is advanced.
In order to enhance the progress of construction operations which involve an extensive use of welding operations, attempts have been made to automate welding processes. The automation of welding processes involves a transfer from a manual welder to a machine of those functions that relate to the parameters affecting formation of a weld. Accordingly, welding machines developed in an effort to obviate manual welding have provided programmed control for torch advancement, welding power-current and voltage, filler wire feed, torch oscillation, and gas flow in the case of a tungsten inert gas (TIG) welding process.
In addition to the above parameters, another aspect in weld formation is that of guidance of the welding torch along the weld seam as the torch is being advanced. As a result of the difficulty in providing accurate automatic tracking control of a weld seam, it has been the case that a welding machine operator has been required to visually monitor the welding process as it proceeds and manually adjust the lateral positioning of the welding torch relative to the weld seam to correct for deviations of the torch from the desired weld path. Accordingly, seam tracking is a basic prerequisite for complete automation of the entire welding process.
Various techniques for tracking a weld seam have been proposed. For example, mechanical seam trackers have been developed which employ a guide probe having one end adapted to be engaged in and follow the joint between two workpieces to be welded and the opposite end mounted for universal movement within a housing mounted to a traveling carriage to which the welding torch is mounted. Also included within the housing is an electromechanical sensor for developing an electrical signal in response to manipulation by the end of the guide probe mounted to the housing. Accordingly, if during the welding process the welding torch begins a lateral deviation from the weld seam to be tracked, the guide probe is altered in its orientation resulting in an electrical signal being developed which can be utilized as a feedback signal to a torch lateral position drive mechanism on the traveling carriage. Mechanical seam trackers, however, are unsatisfactory for use with a welding process which involves a number of filler passes; because, as the last filler pass is made, there is insufficient weld seam sidewall available for the guide probe to track.
It has also been proposed that weld seam tracking be accomplished by monitoring the welding current supplied to the welding torch. In seam tracking apparatus of this type, the welding current is measured, using a shunt resistance to develop a representative signal amplitude. The measured current value is compared with a predetermined reference value and the difference used as a control signal to a mechanism on the traveling carriage for adjusting the lateral position of the welding torch. Changes in the lateral positioning of the welding torch relative to the weld seam effects an alteration of the welding current, leading to the generation of a lateral torch position feedback signal. A problem exists, however, in using torch welding current as the control variable in that changes in the welding current can also be effected by other occurrences, most notably variations in the vertical distance between the welding torch and the workpiece. And even though separate welding torch height adjustment control may be provided, error will nevertheless be introduced into the seam tracking loop.
Another approach to providing non-contact tracking of a weld seam is that disclosed in U.S. Pat. No. 3,918,622. The apparatus disclosed therein is a stationary optical system in which light from a projector lamp is focused by a condenser lens onto a mirror located beneath and directly behind the welding torch and reflected downward to the weld seam. Light, reflected from the weld seam is directed by the same mirror to a lens which focuses the light into an image of the weld seam and directs the image onto a split-screen sensor consisting of two photovoltaic cells separated by a narrow gap. The outputs of the cells are applied as inputs to a differential amplifier circuit, the output of which controls the lateral positioning of the welding torch by selective actuation of a stepping motor and drive mechanism.
If the focused line image is centered on the gap between the sensor elements with equal portions of the image falling on each sensor, the input signals to the differential amplifier are equal and no signal is applied to the stepping motor. If, however, the welding torch begins to deviate from the prescribed welding path, the focused line image moves, placing unequal portions of the image on the sensors, and causes an imbalance in the input signals to the differential amplifier. The sign (i.e. positive or negative) of the differential amplifier output indicates the direction, left or right, of the deviation, and the signal magnitude indicates the extent of deviation.
To completely automate the welding process, there is a need for apparatus which will provide accurate automatic tracking control of the weld seam by a welding torch as it is advanced without any involvement from a manual operator. Such need is particularly acute in the construction of pipelines for transporting crude oil, refined petroleum products, natural gas, and similar products from a point of origin to a processing or distribution center. Such pipelines are constructed by adjoining numerous pipe sections, typically on the order of 30-36 inches in diameter, to form a pipeline several miles in length. Particularly in the construction of offshore pipelines has the need for fully automated welding been felt. However, because of the existence of a relatively narrow weld seam in comparison to the diameter of the pipe sections to be welded, the requirement for multiple passes and a filler wire, and the necessity of mounting the welding torch on a movable carriage, it has heretofore not been possible to suitably provide automatic tracking of a weld seam in fabricating such pipelines. Accordingly, manual guidance of a welding torch along the weld seam has been almost exclusively employed.